Abstract: The Bonnet-Myers theorem states that an n-dimensional complete Riemannian manifold M with Ricci curvature lower bounded by a positive number (n-1)K is compact, and its diameter is no greater than $\pi/\sqrt{K}$. Moreover, Cheng's rigidity theorem tells that the diameter estimate is sharp if and only if M is the n-dimensional round sphere. In this talk, I will discuss discrete analogues of round spheres in graph theory via exploring discrete Bonnet-Myers-Cheng type results. This talk is based on joint works with Cushing, Kamtue, Koolen, Muench, and Peyerimhoff.